Traditional prostheses, leg prostheses as well as arm prostheses, use a socket for connecting the prosthesis to the body. The socket is applied onto the remaining part of the user's limb, the so called “stump”. This means that the transmission of forces between the prosthesis and the user takes place via the remaining soft tissues on the stump. Especially for leg prostheses, which have to carry the body weight of the user, this results in a non-desired so called pumping effect, which means that the soft tissues on the stump inserted into the socket will be pressed together when loaded and be extended when unloaded. In other words there will be an undesired relative movement between the soft tissues and the bones of the user's stump.
This pumping effect causes wear and damage of the tissues. It further results in instability, which may cause a risk for stumbling and leads to a compensating gait with hip-hiking. The gait is more energy consuming and causes stress on the hip, foot and back. It further leads to a low perception for motion and position of the prosthesis.
Socket connection however has several advantages, such as that it permits a high activity level and the risk for fatigue of the material in the prosthesis is low. In order to function well socket connection requires that the stump is relatively long, at which the length of the stump is calculated from the closest joint, which for a below knee amputated is the knee joint and for an above knee amputated is the hip joint. For an arm amputated the length of the stump is in a corresponding way calculated from the elbow joint or the shoulder joint respectively.
It is also previously known to connect a prosthesis to an implant, i.e. an osseointegrated shaft or screw of titanium. The forces between the user and the prosthesis takes place directly from the skeleton system to the prosthesis, and not via the soft tissues of the user. This means that the “pumping effect” is eliminated as well as the problems related thereto such as tissue damages and instable gait. It further gives a good perception of motion and position of the prosthesis. The energy consumption will be lower due to the absence of motion between skeleton and soft tissue. Osseointegrated prostheses may further be used on amputation levels where traditional socket applications are not working properly, mainly amputations with very short stumps, such as high above knee amputations, close below knee joint amputations and close to elbow joint and close to shoulder joint amputations.
The disadvantages are that the rehabilitation period is lengthy since it takes a long time to obtain the necessary strength in the implant in order to be able to put a load on it and an even longer time to reach the maximum activity level. Maximum activity level is still limited and the wearer is for example not recommended to run. A high activity level involves a risk for damage on the part of the implant penetrating the skin, the so called abutment, the bone to which the implant is attached or the metal components adjoining to the implant. Since damage of the bone tissue has to be avoided the dimensions of the implant are normally made weak enough so that when overloaded the abutment will break first. The implant normally has a good strength in axial direction, i.e. when exerted to compressive and pulling forces. It however has a much lower strength when exerted to torsional and bending moments. This is the reason why the freedom of movement for an osseointegrated leg prosthesis is limited and that the user is for example not recommended to run.
DE-A1-100 40 617 discloses an osseointegrated leg prosthesis wherein at least one of the prosthesis components is provided with a material weakening which when a certain load is exceeded will break before the implant or the bone tissue breaks.
U.S. Pat. No. 5,888,215 discloses a lower leg prosthesis wherein the leg stump is relieved from vertical forces acting on it from the socket by means of a mechanism that transfers forces from the prosthesis directly to the tibia. This mechanism comprises an implant extending transversely into the tibia and which is connectable to the socket.
U.S. Pat. No. 6,482,238 discloses an upper leg stump prosthesis connected to a shaft implanted in a truncated femur. An open mesh netting structure covers a portion of the proximal shaft and allows bone tissue to grow into it. A cone-shaped adaptor is provided at the distal end of the shaft and permits the attachment of a substitute for a condyle.
U.S. Pat. No. 3,947,897 discloses an osseointegrated prosthesis, which comprises a circular pressure pad carried adjacent the upper end of the prosthesis for engaging the flesh of the stump. This pressure pad may take up tensile and pressure forces but is not adapted to take up any bending or rotational forces.
U.S. Pat. No. 5,895,429 discloses a leg prosthesis with lockable knee joint, wherein a locking element is provided for locking respectively releasing the prosthesis parts relative to each other. This prosthesis is not osseointegrated.